Electroluminescent device to give negative pictures



United States Patent ELECTROLUMINESCENT DEVICE TO GIVE NEGATIVE PICTURES Frederick H. Nicoll, Princeton, NJ., assigner to Radio Corporation of America, a corporation of Delaware This invention relates to the reproduction or ampliiication of visible light images, and it has for its object to provide a novel and improvedsystem of radiation sensitive elements for this purpose.

Another object of the invention is to provide a simple and efficient electroluminescent energy translating device for reproducing images in reverse or negative form.

Various other objects and advantages will be apparent asthe nature of the invention is more fully disclosed.

This invention is concerned with the elfect called electroluininescence, whereby certain phosphors are excited to luminescence by the influence of an electric field. Phosphors which exhibit this eiect to a substantial de- `gree are known as electroluminescent phosphors. A11 electroluminescent phosphor for example, is zinc sulfide suitably activated with copper, or zinc selenide activated with manganese. When used in a cell, the electroluminescent phosphor is usually embedded in a light transparent dielectric material to form a layer, and a pair of Velectrodes are disposed on the opposite sides of the layer. The present invention relates to the photocontrol of electroluminescence in a system including an electroluminescent element and a photoconductor. In the preferred elemental electrical sypstem disclosed herein, the electroluminescent phosphor is connected in series with a resistance element and a source of alternating or pulsed voltage, and in parallel with the photoconductor. The photoconductor normally has high impedance in the dark, hence in the dark the operating voltage applied across this elemental structure will produce electroluminescent light from the phosphor. When the photoconductor is excited by light, the voltage applied across the phosphor is reduced thus reducing its light output. When an array of these elements is made in the form of a screen, as hereinafter more fully described, then a negative picture will be displayed on the viewing side of the screen "when a `positive picture is projected onto the photoconductor on the input side of the screen. Y i

The invention is described in lconnection with the accompanying single sheet of drawings, in which:

Fig. 1 is a schematic View of a circuit arrangement illustrating one form of a device constructed in accordance with the invention;

Fig. 2 illustrates a device constructed in accordance with the invention for displaying light images;

Fig. 3 is a modification of Fig. 2 utilizing light shielding means.

In Fig. l there is shown a laminated electroluminescent lamp structure including in the order named a transparent foundation member 12 such as a sheet of glass, a transparent conductive coating 14 such as a thin lm of tin chloride, a layer of phosphor 16 of the type which will luminesce under the influence of a varying electric eld, and a conductive member 18. The conductive coating 14 and the conductive member 1S serve as electrodes connecting the electroluminescent phosphor layer 16 into an electric circuit which comprises a photocon- 2,891,169 ce Patented June 16, 1959 ductive cell 20 in series with a resistor 22 and an alternating voltage source 24, the phosphor layer 16 being connected in parallel with the photoconductive cell 20.

The photoconductive cell 20 may be made of any material whose electrical conductivity is variable according to the intensity of the type of radiation. to be employed. An example is cadmium sulde, which, when activated by denite amounts of copper or other metallic impurities, may be made photoconductively sensitive to Visible light, X-rays, infra-red radiation, and other types of radiation. Cadmium selenide is a material which similarly can be made photoconductively sensitive to various types of radiation.

The impedance of the electroluminescent phosphor 16, and that of the photoconductor 20 in the unexcited condition at which no radiation falls on cell 20, is very high compared to the resistance of the resistor 22. Thus, With no radiation on said photoconductor 20, there will be little or no current flow and substantially no voltage drop across the resistor 22; hence, substantially all the voltage of the source 24 will appear across the electroluminescent phosphor `16, causing the latter to give oft" light. With sufficient radiation on the photoconductor 20, however, its resistance will decrease to a value less than that of resistor 22, allowing current to ow, and the resulting voltage drop across the resistor 22 will besuificient to drop the voltage across the electroluminescent phosphor 16 such that the phsphor ceases to luminesce and no light is given oi.

As an example, the electroluminescent phosphor 16 may be a .05 millimeter thick layer having an area of 1 square centimeter, of particles of zinc sulde activated with copper, the phosphor being mixed with a suitable dielectric binding material, such as ethyl cellulose or methyl methacrylate. At 60 cycles alternating current, this area of electroluminescent material has an impedance of about 20 megohms. The resistor 22 may be about 2 megohms and the voltage source 24 may be 500 volts alternating current of 60 cycles per second. Inasmuch as known photoconductive materials have a dark resistivity of the order of l 1010 ohm centimeters and higher, a photoconductive cell 20 of 1 square centimeter and about 0.5 millimeter thick Will have a dark resistance of several hundred megohms. With suicient light incident on the photoconductive cell, its impedance will drop to a value of the order of several thousand ohms or lower.

Hence when an array of these electroluminescent elements is made as hereinafter described, a negative picture will be produced thereby when a positive picture is projected on the photoconductor 20.

In Fig. 2 there is shown a viewing screen 30 comprising a transparent support member `32 having on one surface an electrode in the form ofa transparent conductive coating 34, and on said conductive coating 34 a mosaic made up of a multiplicity of spaced-apart dot groups or clusters. Each cluster comprises an elemental area or dot of electrolurninescent phosphor material 36 and an elemental area or dot of photoconductive material 38 adjacent thereto, A thin layer of electrically conductive, light transmitting material 39, such as a thin semi-transparent layer of silver is coated on the upper surface of each of the dots 36 and 3S. An elemental area or dot of resistive material 40, of appropriate resistivity, is disposed on each conductive layer 39. The other electrode comprises a conductive wire mesh 42, of copper for example, in electrical contact with the upper ends of the resistive dots 4G, the connections being made by silver paste, for example.

With a voltage 44 across the electrodes 34 and 42, and in the absence of a radiant image 48 upon the screen 30, the screen will be illuminated since the photoconductive elements 38 will not be conducting and the corresponding phosphor elements 36 will luminesce due to the voltage drop across them. However, when a lradiant image 48 is focused on the non-viewing side of the screen 30, the photoconductive elements 38 will become conducting to a degree which is directly proportional to the intensity of the incident image radiations 48, and the light from the ycorresponding phosphor elements 36 will become reduced to a similar degree due to the reduction in voltage across them. Hence the picture visible on the screen 32 from the position of the observer 0 is a negative of the picture projected upon the other, or nonviewing, side of said screen. And if a negative picture is projected upon the non-viewing side of the screen 30, a positive reproduction of said picture is exhibited on the viewing side of said screen.

In a fully lighted area the photoconductor will be conducting and the phosphor dot will be completely extinguished and will give oil no light. In dark areas, on the other hand, the photoconductor will be high impedance and a maximum voltage will appear across the phosphor dot which will give off maximum light. In the gray areas the photoconductor will be conductive in an intermediate amount, and the phosphor dot will have reduced voltage across it and will give off only an intermediate amount of light.

If the photoconductor is not shielded from the luminescent light, as shown in Fig. 2, it is possible to retain a picture even after the radiant image is removed. In the device of Fig. 2, for example, it is apparent that some of the light emitted by a phosphor dot 36 will Vfall on an adjacent photoconductive dot 38. This will have an additive effect, over and above the effect caused by the incident radiation 48, in lowering the impedance of the photoconductive material 38. Such feeding of output light will give the device added sensitivity. Moreover, if the amount of light fed back is equal to or greater than the incident light, the device will be a self-sustaining or storage device, and `light will continue to be given off even after removal of the radiant image 48.

Referring to Fig. 3, feedback may be suppressed, if desired, by properly shielding the photoconductor from the light emitted by the phosphor, as by providing a thin layer of light-opaque insulating material 41 over the phosphor dots 36. The layer 41 may be a thin layer of black lacquer. Such a layer may be made thin enough to provide good conductivity through its thickness While at the same time having high resistance in the lateral direction. In this way the dots 36 and 38 will be tied together electrically at their upper surfaces to the semi-transparent conductive layer 39; the dots 36 and 38 will also be tied together electrically at their lower surfaces to the transparent conductive coating 34; and there will Ibe no leakage current between the layer 39 and coating 34 to shunt out either of the dots 36 or 38.

What is claimed is:

1. A radiant energy translating device including a transparent supporting base, a transparent conductive coating thereon, an electroluminescent phosphor material on said conductive coating, a photoconductive element in electrical parallel arrangement with said phosphor material, and means including a series resistor connected to apply a voltage across said phosphor material and said photoconductive .element to cause light to be emitted by said phosphor material.

2. A radiant energy translating device including a transparent supporting base, a transparent conductive coating thereon, an electroluminescent phosphor material on said conductive coating, a conductive member connected to said phosphor material, said conductive coating and said conductive member constituting electrodes connecting said phosphor material in an electric circuit, a photoconductive element connected to said circuit in parallel arrangement with said phosphor material and a resistor and a source of voltage connected in said circuit in series with said parallel arrangement, the impedance of said phosphor material and that of said photoconductive element in unexcited condition being very high compared to the resistance of said resistor, and .the impedance of said photoconductive element decreasing in proportion to 4light radiation thereon.

3. A radiant energy translating device including a transparent supporting fbase, a transparent conductive coating thereon, a mosaic on said conductive coating made up of a multiplicity of spaced-apart elements of electroluminescent phosphor material, a photoconductive element in electrical parallel arrangement with each of said phosphor elements, a resistor electrically in series with each parallel arrangement and means connected to apply a voltage across said series resistor and parallel arrangement to cause light to vtbe emitted by said phosphor material.

4. A radiant energy translating device comprising a luminescent screen including a transparent foundation member; a light transparent conductive layer on a surface thereof; a mosaic on said conductive layer made up of a multiplicity of spaced apart groups of elements, each group including an electroluminescent element., a photoconductive element adjacent thereto, and a resistive element adjacent to said electroluminescent and photoconductive elements; and means connecting said electroluminescent and photoconductive elements of each group electrically in parallel with each other land in series with said resistive element.

5. The invention according to claim 4, and further including light opaque shielding means between said electroluminescent and photoconductive elements.

References Cited in the le of this patent UNITED STATES PATENTS 2,605,335 Greenwood et al. July 29, 1952 2,743,430 Schultz et al Apr. 24,` 1956 2,749,471 Rittner June 5, 1956 

1. A RADIANT ENERGY TRANSLATING DEVICE INCLUDING A TRANSPARENT SUPPORTING BASE, A TRANSPARENT CONDUCTIVE COATING THEREON, AN ELECTROLUMINESCENT PHOSPHOR MATERIAL ON SAID CONDUCTIVE COATING, A PHOTOCONDUCTIVE ELEMENT IN ELECTRICAL PARALLEL ARRANGEMENT WITH SAID PHOSPHOR MATERIAL, AND MEANS INCLUDING A SERIES RESISTOR CONNECTED TO APPLY A VOLTAGE ACROSS SAID PHOSPHOR MATERIAL AND SAID PHOTOCONDUCTIVE ELEMENT TO CAUSE LIGHT OT BE EMITTED BY S AID PHOSPHOR MATERIAL. 